Soap is a universal leak detection method Leak rates ER Leak Detection May 2013 Foamer <0.1 cc/min, very small bubbles looks like foam Bubbler 0.5-10 cc/min clear bubbles 1 cm in dia Vigorous bubbler 10-50 cc/min large bubbles Whistler >50 cc/min can hear the leak What Happens If We Do Nothing? Will It Get Worse? Corrosion of metal valve by a corrosive gas is a process that takes months not minutes for total failure A full 150 lb cylinder contains 150 lbs of liquefied Chlorine Gas The gas specific volume of Cl 2 at 70 o F is 5.39 ft 3 /lb 5.39 ft 3 /lb x 150 lbs = 808.5 ft 3 of gas 808.5 ft 3 x 28,315 cc/ft 3 = 22,892,677 cc At a leak rate of 50 cc/min this will leak for 457,854 minutes (318 days)! Leak rate is measured at cc/sec which is expressed as a power of ten 0.1 cc/sec is 1 X10-1 0.01 cc/sec is 1 X10-2 0.00003 cc/sec is 3 X 10-5 A common cylinder valve thread leaker is 0.1 cc/min (10-3 cc/sec) A valve leaking at 10-6 cc/sec will leak 10 cc in 3 months and will pressurize a typical diaphragm valve to 110 psig behind the vaportight outlet cap. April 2013 ER Gas Leak Detection Page 1
Most likely leak points on a cylinder Evidence of leaks include: Flames Charred or bubbled paint White, Brown or Green fumes Green or Brown liquid dripping down the sides of the cylinder Green or Brown Corrosion Solids Around Connections Black or red deposits Hissing sound Over 99% of leaks if they were to occur during normal transportation are small, < 0.1 cc/min Most Likely Leak Points During Use Cylinder valve outlet connection Pigtail connection to gas panel April 2013 ER Gas Leak Detection Page 2
DOT Definition of a leak There will be no identifiable (without the use of instruments) release of hazardous materials to the environment. ER Gas Leak Detection Simple Methods Soap Solution Applied to Potential Leak Points Alcohol Based Used for Freezing Temperatures Small Leaks of Water Soluble Gases May Not Be Detected Can Interfere with Other Methods Aqueous Solutions Held By Leak Points Ammonia For Acid Gases Hydrochloric Acid For Ammonia or Amines Wet Litmus Paper for Acid or Base Gases Lead Acetate Paper for Hydride Potassium Iodide Paper for Fluorine Gases Leak Detection Method Sensitivity Leak detection fluids (soap solution) 1 x 10-3 to 1 x 10-5 * cc/sec atm He Test Papers (litmus, KI etc.) 1 x 10-3 to 1 x 10-5 * cc/sec atm He Chemical sprays 1 x 10-3 cc/sec atm He Instrumentation 1 x 10-5 to 1 x 10-9 cc/sec atm He Gas Leak Hazards Flammable Toxic Asphyxiant ER Electronic Gas Leak Detectors Key Requirements Must be sensitive to level of interest ppm for Acid and ppb for Hydrides Must be specific for gas of interest with minimum of interferences Must be capable of working when needed, Battery operated Must be fast and be able to pinpoint leak point Accuracy/Precision Not as critical as other parameters Continuous Extraction rather than diffusion Ergonomically designed for ER use in PPE Problem is most gas detectors will become saturated if the leak is more than a foamer April 2013 ER Gas Leak Detection Page 3
General gas meter, oxygen and flammable with 2 additional cells (H 2 S, Cl 2, CO, etc) Flammable gas detection is typically catalytic bead Principle : Hot wire catalytic platinum filament oxidizes flammable gases or vapors at lower levels than they would normally oxidize in air. causing an increase in temperature and resistance Thermal conductivity detectors measure the change in thermal conductivity of the air drawn across a resistance cell. The larger the difference in thermal conductivity of the gas and air, the greater the sensitivity. To zero, air is drawn across the detector and the sensor is adjusted for no alarm The most common chemical detection technology used in industry is electrochemical. Electrochemical sensors are typically diffusion type, which have a liquid or gel electrolyte that reacts with the gas. For ER the electrochemical gel sensors are the most practical The gas specific sensor can be inserted depending on the type of gas leaking. Key is having a small trickle charging case which keeps the sensors in equilibrium and immediately ready for use April 2013 ER Gas Leak Detection Page 4
Personal Monitors, electrochemical gel type Electochemical Detectors will detect the same chemical family. Metal Hydride family Papertape Historically the more stable and sensitive chemical detection technology uses chemically impregnated paper tape which will react with the gas. This technology can only be used in an extractive mode with the air pulled through the paper tape. The gas reaction causes a discoloration on the tape and the concentration is measured colorimetrically by the reflection from a light source on this spot At a typical gas cabinet ventilation flow of 381 cfm (10.8 m 3 /min) a pure silane leak of 63 cc/min will cause a 5 ppm alarm Gas Detector Tubes, single use Area sampling. Must consider response time. Systematic approach to air monitoring Break room/building into quadrants April 2013 ER Gas Leak Detection Page 5
Sample each quadrant at 3 heights Ceiling Chest Floor A confined space should also be sampled at the 3 heights When monitoring confined spaces, it is equally important to sample at a distance from the opening because an intrusion near the entrance may result in a false oxygen level reading April 2013 ER Gas Leak Detection Page 6
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